I have a yahoo.co.uk email account. Starting a few days ago any email I send to someone with an O2 account comes back with a failure notice and a message including "550 mail not accepted from blacklisted IP address". These are messages to friends who I am fairly sure have not blacklisted me.
Some other messages have recently come back with a failure notice and the message "550 Blocked".
Each of these messages comes with a link to a Spamcop page but I'm afraid I can't work out why it is happening or what is going on.
Can anyone help or explain what is happening please?